Generally, the operation of a conventional apparatus for calibrating radiation measurement instruments comprises the following steps: a radiation measurement instrument that is to be calibrated in a radiation field produced from a standard radiation source; recording and analyzing the readout values of the to-be-calibrated instrument so as to obtain a calibration factor or test parameter for the to-be-calibrated instrument. However, it is noted that using the aforesaid conventional calibration apparatus, the interference of the background radiation and scatter radiation toward the readout values of the to-be-calibrated instrument is generally unpreventable, and consequently, not only the radiation measurement accuracy of the radiation measurement instrument that is calibrated using the conventional calibration apparatus without necessary correction is questionable, but also the safety of radiation professionals may be endangered by the incorrect radiation readout. Thus, it is in need of an improved radiation measurement instrument calibration facility that is capable of effectively reducing the interference of the background radiation and scatter radiation toward the instrument and thus uplifting the measurement accuracy. In addition, the establishment of such a facility can also to be used for providing a suitable environment for performing performance test, calibration and experiment upon the radiation measurement instruments.
Please refer to FIG. 1, which is a schematic diagram showing a conventional apparatus for calibrating radiation measurement instruments. As shown in FIG. 1, the conventional calibration apparatus is installed inside a laboratory 1, while allowing a to-be-calibrated instrument 11 to be disposed on a cart 12 that is mounted on a rail 13 so as to be moved along therewith, and thus enabling the to-be-calibrated instrument 11 to move to a specific location for allowing the same to be irradiated by a primary radiation beam emitted from a radiation source 141 of irradiator 14. It is noted that, operationally, the readout values of the to-be-calibrated instrument 11 are interfered by the background radiation 15 and scatter radiation 16 of the primary radiation beam inside the laboratory 1. In the regulation specified in ISO 4037-1 Standard (1996), the amount of scatter radiation 16 should not exceed 5% of the primary radiation beam. However, the conventional calibration apparatuses are not able to ensure to lower their scatter radiation to meet the ISO 4037-1 (1996) requirement due to the space limitation and structural design of the laboratory where they are installed. In addition, although the background radiation 15 of most conventional calibration apparatuses can be neglected while operating in medium and high dose rate radiation field, but while operating in low dose rate radiation field, the measurement accuracy can be severely affected by the background radiation 15 since its amount can achieve more than 20% of the primary radiation beam. Thus, the improved radiation measurement instrument calibration facility of the present invention is designed to overcome the aforesaid shortcomings in view of enhancing the accuracy of measurement or calibration for the instrument.